Effect of Stem Positioning on Biomechanical Performance of a Novel Cementless Short-Stem Canine Total Hip Implant.

OBJECTIVE: The aim of this study was to evaluate the effect of stem positioning on the biomechanical performance of a novel, collared, short-stem total hip implant under compression and torsion ex vivo. STUDY DESIGN: Six canine cadaveric femurs were implanted with a collared short-stem femoral implant. Canal flare index (CFI), stem angle, absolute and relative cut heights and relative size were measured radiographically and used as independent variables. Biomechanical performance of the construct was evaluated using physiologic loading (loading) and supraphysiologic loading (failure) protocols. RESULTS: During loading protocols, compressive stiffness was influenced by absolute cut height (p = 0.018). During failure protocols, peak torque was influenced by CFI (p = 0.004) and craniocaudal relative size (p = 0.005). Peak load and torsional stiffness were not impacted by any of the radiographic variables (p > 0.05). Three of six femurs developed longitudinal fractures originating at the medial calcar at the time of failure. CONCLUSION: The biomechanical performance of the collared short-stem implant was positively impacted by preserving more of the femoral neck, having a higher CFI and using a smaller implant size relative to the femoral neck isthmus.

Radiographic and Biomechanical Assessment of Three Implant Designs for Canine Cementless Total Hip Replacement.

OBJECTIVE: The aim of this study was to evaluate the relationship between radiographic fit/fill measurements and biomechanical performance of three canine cementless total hip implant designs using an in vitro biomechanical testing protocol that replicates compression and torsion. STUDY DESIGN: Eighteen (six/group) canine cadaveric femurs were implanted with one of three cementless total hip implant designs: (1) collarless, (2) collared or (3) lateral bolt stems. Femoral length, canal flare index (CFI), canal fill, stem fit, stem level and stem angle were measured as independent variables. Biomechanical performance was tested using physiological, non-destructive gait loading (loading protocols) and destructive testing (failure protocols). RESULTS: During loading protocols, compressive stiffness was influenced by stem level (p < 0.05) and torsional stiffness was influenced by stem level and CFI for collarless stems (p < 0.05). During failure protocols, peak load was influenced by mediolateral (ML) stem angle (p < 0.05) and CFI (p < 0.01) for collarless stems and CFI for lateral bolt stems (p < 0.05). Peak torque was influenced by ML stem angle, craniocaudal stem angle and CFI for collarless stems (p < 0.05) and average ML fill for collared stems (p < 0.05). CONCLUSION: Biomechanical performance of collarless stems in cementless hip arthroplasty is more impacted by radiographic fit/fill than lateral bolt and collared stems. As a result, collarless stems may be more dependent on preoperative fit and intraoperative precision.

A Biomechanical Comparison of Four Hip Arthroplasty Designs in a Canine Model.

OBJECTIVE: The aim of this study was to develop an in vitro biomechanical protocol for canine cementless hip arthroplasty that represents physiological gait loading (compression and torsion) and to evaluate if three alternative implant designs improve fixation compared with the traditional collarless, tapered stem in the clinically challenging case of moderate canal flare index. STUDY DESIGN: Twenty-four (six/group) laboratory-prepared canine constructs were tested using a simulated gait and overload (failure) protocol. Construct stiffness, failure load/displacement and migration were measured as outcome variables. RESULTS: Simulated gait loading did not show any significant differences between implant types for peak displacement, peak rotation, torsional stiffness, subsidence or inducible displacement. The collared and collarless stem groups were stiffer in compression compared with the collarless with a lateral bolt and short-stem groups. Increasing the loading above simulated gait showed significant reductions in compressive and torsional stiffness for all implant constructs. Despite the reductions, the short-stem group showed significantly higher stiffness compared with the other three groups. CONCLUSION: Peak failure loads (compressive and torsional) in this study were approximately four to seven times the simulated gait loading (430 N, 1.6 Nm) regardless of implant type and highlight the importance of limiting activity level (trotting, jumping) following hip replacement in the postoperative period and during the osseointegration of the implant.